Menge and colleagues performed experiments in which they exposed some populations of mussels to sea stars and excluded sea stars from other mussel populations. Suppose that the sea star population consists of 20 individuals and the two populations of mussels are of the same size prior to the mussel treatment. In the treatment condition there are 500 individuals without sea stars and 100 individuals with sea stars. What is the per capita interaction strength of sea stars on mussels?